Look through the ChangeLog online
The ChangeLog is divided into four sections "Breaking Changes", "Features", "Deprecation" and
"Important". Before upgrading you should at least take a look at the sections "Breaking Changes"
and "Important" - changes described in those areas might affect your website.

The detailed information contains a section called "Affected Installations" which contains hints
whether or not your website is affected by the change.

You can also find the ChangeLog in the Install Tool at "Upgrade Analysis". Here you can mark files
as read after checking them, so you get a ToDo list for the upgrade.